1. Field of the Invention
The invention relates generally to the field of seismic imaging of the Earth's subsurface. More specifically, the invention relates to imaging of the Earth's subsurface using passive seismic sensing techniques.
2. Background Art
Passive seismic emission sensing techniques include detecting seismic signals from within the Earth's subsurface. As contrasted with conventional controlled source seismic exploration techniques (wherein a seismic source is actuated near the Earth's surface), in passive seismic sensing, the seismic signals are generated by seismic events taking place within the Earth's subsurface. The subsurface seismic events may be naturally-occurring or may be induced by man-made activities. The seismic signals are detected by an array of seismic sensors positioned at or near the Earth's surface generally above a target volume within the Earth's subsurface. Applications for passive seismic emission tomography include, for example, determining the point of origin of micro-earthquakes caused by movement along geologic faults, i.e., breaks in rock layers or formations, monitoring of fluid movement within the Earth's subsurface, and monitoring of fluid injected into the Earth's subsurface, e.g., in a hydraulic fracturing process or in monitoring movement of a fluid contact in a subsurface reservoir.
In some cases it may be undesirable to use conventional controlled source seismic techniques for evaluating the Earth's subsurface, for example, if a particular area is environmentally sensitive so as to make access and use of seismic sources unsafe or impracticable. There is a need for passive seismic methods that can make three dimensional images of the Earth's subsurface similar to those obtained using conventional controlled source seismic exploration techniques.
One technique for passive seismic imaging is known as passive seismic transmission tomography (“PSTT”). PSTT is known in the art for producing three dimensional (“3-D”) images of the subsurface. PSTT can produce such images of the Earth's subsurface using observed travel time of seismic signals originating from micro-earthquakes occurring below or beside a target image volume in the subsurface. An array of seismic sensors, typically three-component geophones or other particle motion sensors is disposed near the Earth's surface (or on the bottom of a body of water in marine surveys) above the target volume in the subsurface. Typical imaging areas for such an array may be on the order of 300 to 1500 km2. The three-component seismic sensors may be placed 10 to 30 or more meters below the Earth's surface in land-based surveys to avoid the noisy surface environment. Selected numbers of such sensors may be arranged in groups coupled to various stations at the Earth's surface. The stations may store recorded signal data locally, but may also be linked to a processing center by some form of telemetry. Travel time inversion is used to estimate the compressional and/or shear wave velocity distribution in the target volume from the recorded signals. As more events are observed, the velocity distribution can be estimated to a finer resolution. While effective, PSTT may provide ambiguous results or may require very long signal recording time to generate unique results.
There continues to be a need for improved methods of imaging the Earth's subsurface using passive seismic acquisition techniques.